See the function link_advanced_meta_box() in wp-admin/includes/meta-boxes.php:
<td><select name="link_rating" id="link_rating" size="1">
<?php
for ( $r = 0; $r <= 10; $r++ ) {
echo '<option value="' . $r . '"';
if ( isset($link->link_rating) && $link->link_rating == $r )
echo ' selected="selected"';
echo('>' . $r . '</option>');
}
?></select> <?php _e('(Leave at 0 for no rating.)') ?>
The value is hard-coded.
You could deregister the metabox 'linkadvanceddiv' and register your own instead.
Be aware the link manager has no future. I would use a custom post type instead with specialized meta data handling.